Implantable spinal prosthetic devices are well known in the art. Presently, the primary method used to remediate severe disc disease, spinal instability, discogenic pain, and/or spinal stenosis, is by surgical spinal fusion. In the spinal fusion procedure, two or more adjacent vertebrae are displaced, the spinal discs in between the vertebrae are removed by dissection, and crushed bone material is inserted between the two vertebrae; the bony material promotes the growth of new bone in the intervertebral space. The bony fusion material may be harvested intra-operatively from the patient's iliac crest or, alternatively, banked bone may be used. Since the fusion depends upon the ingrowth of new bone which takes months, mechanical means are necessarily incorporated at the time of surgery to maintain the stability and proper spacing between the vertebrae so as to permit the patient to carry normal loads imposed on the patient's spine during normal activities. Once the affected vertebrae are fused, that spinal segment will no longer take part in normal flexing, extending and twisting movements; higher stress loads will subsequently be imposed on discs and vertebra above and below the fused vertebral segment, often leading to the patient developing transition syndrome.
An important goal of spinal disc prosthesis implantations is to obviate the loss of normal biomechanics and range of motion associated with surgical fusion of a diseased spinal segment. Lordosis is an important element of the biomechanics of the spine, especially in the lumbar spine. While the lumbar vertebrae could be articulated in such a way that they form a straight vertebral column, this is not the shape assumed by the normal lumbar spine when a person is in the upright posture. This is because the sacrum, on which the lumbar spine rests, tilts forward so that its upper surface is inclined downwards and forwards. The size of this angle, with respect to a horizontal plane of the body, has a value in the range of about 40-45 degrees and increases by about 8 degrees upon standing. A straight lumbar spine would have to be inclined forward to articulate with the sacrum. In order to restore a normal upward orientation and to compensate for the normal inclination of the sacrum, the intact lumbar spine must assume a curve that is known as the lumbar lordosis. The shape of lumbar lordosis is achieved as a result of several factors. One of the main factors is the shape of the lumbar discs, and particularly the L5-S1 lumbosacral intervertebral disc. The L5-S1 lumbosacral disc, more than other lumbar intervertebral discs, is substantially wedge-shaped. Typically, the posterior disc height is about 6 or 7 mm less than its anterior height. The angle formed between the bottom of the L5 vertebrae and the top of the sacrum (S1) is found to vary from person to person in a range of roughly 5 to 30 degrees, with an average value of about 16 degrees.
One important advantage that derives from the lumbar lordosis is resilience to compressive forces and shocks. In a straight lumbar spine, axial compressive forces would be transmitted through the vertebral bodies and intervertebral discs and the only mechanism to protect the lumbar vertebra would be the shock-absorbing capacity of the intervertebral discs.
In a normally curved lumbar spine, compressive forces are transmitted through the posterior ends of the intervertebral discs while the anterior ends of the vertebral bodies tend to separate. Compression tends to accentuate the lumbar lordosis, which tendency tenses the anterior ligaments, which in turn resists the accentuation. Thus some of the energy of the axial compressive force is diverted into the stretching of the associated ligaments instead of being transmitted directly to the next vertebral body. In order to restore relatively normal biomechanical relationships to the vertebral column having structural derangements severe enough to require prosthetic spinal disc implantation, the prosthesis ought to provide for and replicate—as much as possible—the normal lordosis found in the healthy spine.
Axial compression is the movement that occurs during weight-bearing in the upright posture, or as a result of contraction of the longitudinal back muscles. During compression, intervertebral discs undergo an initial period of rapid creep, deforming about 1.5 mm in the first 2 to 10 minutes depending on the size of the applied axial load. Subsequently, a much slower but definite creep continues at about 1 mm/hour. Depending on age, a plateau is attained by about 90 minutes beyond which no further creep occurs. It is therefore important to incorporate this gradual accommodating compression—this cushioning—of the intervertebral disc to axial loads as part of the effort to restore and replicate normal vertebral biomechanics as much as possible.
During the axial rotation of an intervertebral joint inherent in twisting movements, the normal intervertebral disc resists torsion more than bending. Normally, the stress-strain curves for torsion rise steeply in the range of 0 to 3 degrees of rotation; beyond 3 degrees very large forces have to be applied to rotate the disc further. The risk of disc element failure increases substantially as the amount of rotation approaches 12 degrees, suggesting that 12 degrees is normally the maximal range of rotation. Thus, in order to replicate normal spine movements, an implanted prosthetic spinal disc ought to permit at least 3 degrees of rotation and preferably between 8 and 12 degrees of maximal rotation. None of the currently available disc prostheses provide for anything close to this amount of rotation.
Commonly used implantable spinal prosthetic devices include semi-rigid elastomeric filler materials that are sandwiched between two layers of some bio-compatible metal. The upper and lower plate surfaces typically have multiple spikes for their fixation to the vertebral end plates. Other similar devices offer means to screw the upper and lower plates to the co-joining vertebrae and some also include plates treated to promote bone growth into them. A few of the newer devices permit a small amount of articulation between the vertebrae but the extent of flexing and twisting is quite limited; furthermore, the elastomeric materials and their bonding agents in these devices have a disappointingly limited longevity. Ideally, a spinal disc prosthesis should last 30 to 40 years and be able to withstand approximately two million compression cycles per year.
It is a purpose of this subject invention to provide an implantable spinal disc prosthesis assembly comprising a combination of selectable modular components that has a long life expectancy, a negligible rate of failure and/or complications, and provides for maximal articulation in all normal physiological planes of movement within the spine. More particularly, the subject spinal disc prosthesis allows for tilting from side-to-side, rotation such as with twisting movements, and compression along a primary axial direction to absorb and transmit axial loads typical for normal activities.